This is a complete lesson for teaching your students how to find the greatest common factor of two numbers. The greatest common factor is the largest number that is a factor of both numbers. Consider 135 and 72. The prime factors of 135 are 3, 3, 3, and 5. The prime factors of 72 are 2, 2, 2, 3, and 3. The common prime factors are 3 and 3, and 3 x 3 = 9. The greatest common factor is therefore 9. You should teach this lesson after your students have mastered finding prime factors of 2, 3, 5, and